What is Superheat and Subcooling? The Ying and Yang of Air Conditioning


Together they let you take a peek into the inner workings of the magic known as refrigeration, separately they’re about useless.

It seems everyone is fascinated with super heat. They’re quick to add refrigerant if the superheat is “high”, or start cranking on the TXV adjustment if it’s “low”.
First, “high” and “low” are pretty darn arbitrary references. What the heck is high? What’s “low”? What’s just right? (sounds like Goldy Locks and the three technicians….)
Some Techs think 8° is good, others like 18°, and still others aim for 13°. So, which one is correct? I’ll tell you what one is correct; the one listed by the machine manufacturer.
Some specify 5°, others 13°, and some 20°. On systems with a fixed metering device the superheat is based on actual system load. Is there a rule-of-thumb for super heat?
The rule-of-thumb for superheat is the same as all the other rules-of-thumb; “Rules of thumb are for jackasses”.
That being said, a typical commercial TXV maintains 10° superheat under normal conditions and a residential TXV can vary between 3° to 15° depending on load.
However, before we dive into what the superheat should be, we have to understand that superheat doesn’t tell you jack until you know what the Subcooling is.
Systems equipped with a TXV or EXV are charged based on subcooling. Superheat is used to determine if the metering device is operating properly AFTER you determine the charge is correct.
In fact, superheat doesn’t mean squat until the air flow / water flow is correct. If you’re starting to get the idea that superheat is one of the last things you check on a system, not the first, you’re right!
We are not telling you not to check it or that it doesn’t matter; we’re saying it’s dependent on too many other variables to be useful on its own.
Want to learn more? Check out some of our other articles.

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Patrick is Zen HVAC’s diagnostic and training guy. Patrick started in the trade the day he left technical school and never looked back. He's served in various technical and training roles in the HVAC industry but specializes in system troubleshooting and diagnostics, retro commissioning, and technical training. His moto: If I can understand it, anybody can. Patrick uses the Zen common sense approach to teach Patrick’s Likes- His Wife, kids and dog. Old pickup trucks. Hiking. The industrial Revolution. Patrick’s Dislikes- Taking work too seriously. Anything unintuitive. Emoticons :( Patrick’s Favorite famous person- Theodore Roosevelt “I am only an average man, But I work harder at it than the average man" Famous Patrick Quote- “Well, that was stupid of me”

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